1. Field of the Invention
This invention relates to a method for the manufacture of high-frequency dielectric ceramics, and more particularly to a method for the manufacture of high-frequency dielectric ceramics having high unloaded Q and low loss.
2. Description of the Prior Art
Generally dielectric resonators and dielectric substrates which are used in circuits for signals in the region of high-frequency waves such as microwave and millimeter wave are desired to be made of dielectric ceramics having high unloaded Q. Incidentally, the frequencies used in communications are suddenly growing in level in recent years. The satellite broadcast using waves in the SHF band is entering the stage of practical utilization. Thus, the desirability of developing dielectric ceramics possessed of still higher unloaded Q and low loss has come to be urged with growing enthusiasm.
The conventional low-loss dielectric ceramics for use with high-frequency waves have unloaded Q at levels falling in the range of 3,000 to 7,000. Only in very recent years has the manufacture of low-loss dielectric ceramics having unloaded Q at levels exceeding 10,000 materialized.
Known BaO-MgO-Ta.sub.2 O.sub.5 type dielectric ceramics, particularly those containing no additive, have unloaded Q at levels of about 4,000 (see Japanese Patent Application Laid-open SHO 53(1978)-60,544). Even the BaO-MgO-Ta.sub.2 O.sub.5 type ceramics incorporating additive elements have unloaded Q at low levels, such as 4,360 in the case of ceramics of the composition of 3BaO.xMgO.(1-x)-znO.Ta.sub.2 O.sub.5 disclosed in Japanese Patent Application Laid-open SHO 54(1979)-77,000 and 4,090 in the case of ceramics of the composition of 3BaO.xMgO.(1-y)Nb.sub.2 O.sub.5.yTa.sub.2 O.sub.5 disclosed in Japanese Patent Application Laid-open SHO 54(1979)-71,400. Thus, these ceramics are not suitable as materials for communications in the SHF band.
For the purpose of improving the BaO-MgO-Ta.sub.2 O.sub.5 type ceramics in unloaded Q, there has been proposed a method which comprises adding a small amount of Mn to an oxide of the Perovskite configuration consisting preponderantly of Ba(Mg.sub.1/3 Ta.sub.2/3)O.sub.3 and sintering the resultant mixture (see Japanese Patent Application Laid-open SHO 53(1983)-206,003). This method indeed, is capable of producing dielectric ceramics having unloaded Q at levels exceeding 10,000. The incorporation of such an extraneous element as Mn, however, entails the possibility that the process of manufacture of ceramics will be complicated and, in the course of the addition, impurities unnecessary and even detrimental to the characteristics of ceramics will find their way into the ceramics. Further, this method has the disadvantage that for the sake of quality control of products, special care must be paid to the control of the amount of addition and to the uniform distribution of added Mn. This method, therefore, is unfit for mass production of the ceramics under discussion. Incidentally, the aforementioned BaO-MgO-Ta.sub.2 O.sub.5 type dielectric ceramics are invariably manufactured by the conventional method of firing, i.e. by firing a green compact of a prescribed composition at a temperature roughly in the range of 1,000.degree. to 1,500.degree. C. In this case, no due respect has been accorded to the course of temperature elevation to the prescribed firing temperature. For feat of breakage of ceramics by sudden heating, it has been customary heretofore to place a green compact in the furnace and heat the green compact generally at a temperature increasing rate of 2.degree. to 20.degree. C./minute so as to elevate the internal temperature of the furnace gradually from room temperature to the prescribed temperature. The conventional firing method itself has the disadvantage that the course of temperature elevation which calls for a much as several hours' time poses itself as a hindrance to the improvement of production efficiency.
In accordance with the conventional method, BaO-MgO-Ta.sub.2 O.sub.5 type dielectric ceramics having unloaded Q at high levels cannot be easily obtained unless such an additive element as Mn is incorporated therein as described above. The method involving the use of Mn or other similar additive element inevitably entails various drawbacks such as, for example, complication of the process of manufacture. Moreover, the conventional firing method has not been free from the problem that the lengthy course of temperature elevation impairs the efficiency and productivity.